The knee joint is made up of the distal end of the femur, which articulates with the proximal end of the tibia, and the patella, which slides in a groove on the femur. Ligaments attach the femur and tibia to provide stability.
The tibiofemoral joint at the knee in humans and animals allows for flexion (bending movement that decreases the angle between the femur and tibia) and extension (straightening movement). In addition to flexion and extension, motion of the knee is both rotational and translational. The femoral condyles both roll and glide as they articulate with respect to the tibial plateaus. The tibiofemoral joint has a ‘screw-home’ mechanism wherein during knee extension the tibia rotates externally, and this motion is reversed when the knee flexes, providing external and internal rotation within the knee joint.
In humans, knee joint replacements are commonly used for treatment of a variety of clinical conditions and there are a large number of designs to choose from. This is not true for the treatment of canine or feline populations where in the majority of cases euthanasia is usually a solution and for a few where there are a limited number of designs in the market that are available, these all have a very limited application and clinical function. In contrast to human applications the usage and loadings in four legged animals are considerably different, thus requiring a different approach to knee joint replacement.
The rotating hinge knee mechanism is a human knee joint replacement mechanism that was designed to provide a stable total knee reconstruction when the intrinsic stability of the knee has been lost as a result of a severe soft tissue compromise. Prior art rotating hinge knee designs have a transversely oriented hinge axis for flexion-extension motion and a vertically oriented post-in-channel axis for internal and external rotation by means of a post extending down from a bearing component being received in a channel in the tibial component. The post-in-channel design also allows distraction up to the limits imposed by soft-tissue tension. Component dislocation due to distraction is prevented only by the restraint of the soft-tissue envelope and dislocation is a potential problem with this knee replacement design. The amount of distraction required for implant dislocation is related to factors such as the length of the post and degree of taper of the post etc.
The canine and feline knee is anatomically comparable to the human knee. The bone contour, the ligamentous stabilizers (passive restraints) and the muscular support (active restraints) are similar. However, the forces acting on the knee are different for canines and felines due to the normal bent-knee stance of canines and felines, compared to the upright stance in humans.
There is a need for a knee joint replacement prosthesis that is suitable for use in quadrupeds, whose requirements for a knee joint replacement are different from those for bipeds. Features of such a knee joint replacement prosthesis may of course be suitable for knee joint replacement in a biped also.